Tbx1 is a highly conserved T-box-encoding transcription factor. Loss of function of Tbxl in mice is associated with severe developmental defects of the external, middle and inner ear, as well as other developmental abnormalities. TBX1 is thought to be a critical gene in the pathogenesis of de122qll/DiGeorge syndrome (DGS). Morphological abnormalities of the external ear and hearing impairment (conductive or sensorineural) affect the majority of patients. The external and middle ear defects in the mouse model are consistent with the requirement of Tbxl for the development of the pharyngeal arches but the inner ear defects are of unknown origin. Preliminary data underline the requirement of Tbxl for the growth of the otocyst and for the formation of the cochlear duct and semicircular canals. Because of the fundamental importance of the affected developmental processes, we propose a genetic dissection of the function of Tbxl in the inner ear. The first aim of the project is to establish the mechanism by which Tbxl loss of function blocks otocyst morphogenesis. We hypothesize that this is due to growth failure, death or fate change of a subpopulation of otic epithelial cells. We will use chimera and cell fate analyses to address this hypothesis. The second aim is to understand whether Tbxl expression is required in the otic epithelium, periotic mesenchyme or both. We hypothesize that Tbxl is required cell-autonomously in the otic epithelium and we will dissect this function from a possible non-cell autonomous role in the mesenchyme using tissue-specific mutation of the gene. The third aim is to establish whether quantitative reduction of Tbxl RNA message can cause morphological, molecular and/or functional abnormalities of the inner ear. We hypothesize that the function of Tbxl in inner ear development is dosage-dependent and we will use a hypomorphic Tbxl allele to test this hypothesis. In particular, we would like to understand whether Tbxl dosage reduction could cause hearing impairment, a common clinical finding in DGS patients.